tests: support overriding disko config

[Enzime]

The goal of this PR is to add an interface for end users to be able to override the disko config for disko-based tests like system.build.installTest.

I modified some examples to intentionally fail so that we can add test-specific overrides in the same file to show end users examples of overriding the disko config just for tests.

I chose to not make makeDiskoTest aware of disko-config.disko.tests.extraDiskoConfig to simplify the merging logic, however that means callers of makeDiskoTest need to set extraDiskoConfig from disko.tests.extraDiskoConfig if they're using standalone files like the tests inside this repo.

I'm not really sure if this PR is the right way to implement this as currently it feels like makeDiskoTest is just a poor man's module system:

disko/lib/tests.nix

Lines 44 to 80 in 9ab9637

	makeDiskoTest =
	{ name
	, disko-config
	, extendModules ? null
	, pkgs ? import <nixpkgs> { }
	, extraTestScript ? ""
	, bootCommands ? ""
	, extraInstallerConfig ? { }
	, extraSystemConfig ? { }
	, efi ? !pkgs.hostPlatform.isRiscV64
	, postDisko ? ""
	, testMode ? "module" # can be one of direct module cli
	, testBoot ? true # if we actually want to test booting or just create/mount
	}:
	let
	makeTest' = args:
	makeTest args {
	inherit pkgs;
	inherit (pkgs) system;
	};
	# for installation we skip /dev/vda because it is the test runner disk
	importedDiskoConfig =
	if builtins.isPath disko-config then
	import disko-config
	else
	disko-config;
	diskoConfigWithArgs =
	if builtins.isFunction importedDiskoConfig then
	importedDiskoConfig { inherit lib; }
	else
	importedDiskoConfig;
	testConfigInstall = testLib.prepareDiskoConfig diskoConfigWithArgs (lib.tail testLib.devices);
	# we need to shift the disks by one because the first disk is the /dev/vda of the test runner
	# so /dev/vdb becomes /dev/vda etc.
	testConfigBooted = testLib.prepareDiskoConfig diskoConfigWithArgs testLib.devices;

and as I'm merging values manually, it makes it feel like that even more:

disko/lib/tests.nix

Line 79 in 6b98b81

extendedDiskoConfigWithArgs = lib.recursiveUpdate diskoConfigWithArgs { disko = extraDiskoConfig; };